The knee joint functions as a knee capable of swinging an upper member and a lower member. One representative example of an artificial leg including such a knee joint is an artificial thigh (or an above knee prosthesis). The artificial leg including the knee joint normally comprises a resistance force (or a drag) generating means, such as a pneumatic cylinder, a hydraulic cylinder or a spring cylinder, for effectively performing swinging motion. From the standpoint of size reduction or simple structure of the artificial leg, the hydraulic cylinder and the spring cylinder are more preferable than the pneumatic cylinder. On the other hand, the pneumatic cylinder has such a feature that a repulsive force can be obtained after the knee is bent maximum.
For example, Patent Document 1 discloses an artificial leg including a pneumatic cylinder; Patent Document 2, an artificial leg including a hydraulic cylinder; and Patent Document 3, an artificial leg including a spring cylinder, respectively.    Patent Document 1: Japanese Patent Laid-Open Gazette No. 2001-137268    Patent Document 2: Japanese Patent Laid-Open Gazette No. 2002-58689    Patent Document 3: Japanese Patent Laid-Open Gazette No. 1980-130657
Since the artificial leg including the knee joint has an upper and a lower member which are swingable, prevention of any accidental knee bending is desirous for its wearer. Patent Document 4 shows a technology for preventing knee bending wherein swinging motion is automatically locked/unlocked at walking by generating a braking effect in accordance with application of the wearer's load.    Patent Document 4: Japanese Utility Model Publication Gazette No. 1980-32574
However, since such a technology for preventing knee bending uses the wearer's load, it cannot be effectively applied, for example, to a case where the wearer walks on a snow-covered road or an uneven ground. In order to reliably prevent accidental knee bending at walking on a snow-covered road or an uneven ground, a locked condition should be achieved when the knee is in an extended condition. The expression “the knee-extended condition” refers to a state wherein the upper and lower members are extended straight (Patent Documents 5 and 6) and a state wherein the upper and lower members are bent in a dogleg shape (Patent Document 7).    Patent Document 5: Japanese Patent Laid-Open Gazette No. 1987-87149    Patent Document 6: GB2099708A    Patent Document 7: Japanese Patent Laid-Open Gazette No. 1997-75382